This invention relates to an apparatus for the electrostatic charging of photo-conductive semiconductor layers, in particular photo-electrostatic zinc oxide layers. Specifically, the invention is intended for use according to the so-called Corton method as discussed in German Utility Model Application No. G 7,404,433 published Sept. 11, 1975.
This Corton principle which is possible with specific photo-conductive semiconductor layers which are able to store the image information for a sufficient period of time prior to charging has advantages for numerous reasons over the conventional method in which the exposure is effected after charging. The advantages of an exposure prior to charging are mainly provided by the substantially more stable latent conduction image which is produced after exposure as compared with the latent charge image which is present after charging and exposure. The latter should be produced only shortly prior to development (toning).
The Corton method also has the advantages to conduct exposure independent of the charging and development. So for instance the production of the latent conduction image may be effected substantially quicker or slower than the charging or development. In the registering field for instance paper velocities of up to 6 m/sec occur at which a latent conduction image may be produced. The production of the latent conduction image therefrom would only be possible with very high expense at this speed. Via a buffer cassette, this exposure may be made visible with a few seconds delay in using a Corton apparatus.
No further statements are here to be made on the production and the structure of the latent conduction images. It is stated for an understanding of the apparatus according to the invention, however, that the structure of the conduction image is dependent on numerous factors such as exposure time and intensity and effects which are effective prior to, during and after exposure (e.g. stray light, heat etc.).
It has been ascertained that the quality of the developed image, i.e. rendering visible the conduction image composed so differentiatedly, to a high degree depends on the nature of conversion into a corresponding charge image. In other words, this means that according to the Corton principle very precisely neither too much nor too little charge is to be applied to the exposed photoconductive semiconductor layer. Too much charge would partially or entirely extinguish the latent conduction image again. Too little charge would result in too less a potential difference between exposed (conductive) and nonexposed (insulating) spots, and this has as a result a too weak blackening of the exposed spots by a reversal toner.
Contrary thereto it was not so important with the conventional method of charging prior to exposure for the quality of the developed image whether the layer was overcharged or not prior to exposure.